1. Technical Field
An embodiment of the invention relates to a control apparatus for a Continuously Variable Transmission (CVT), particularly to an apparatus that controls axial thrust of the CVT.
2. Related Art
A CVT control apparatus has been proposed in an U.S. Patent Application No. 2002/0155910 A1 (partly corresponding to a Japanese Laid-Open Patent Application No. 2003-65428), in which a thrust ratio between the drive (input) pulley axial thrust and driven (output) pulley axial thrust of the CVT is determined, a peak of the thrust ratio variation caused by a change of the driven pulley axial thrust is detected from the thrust ratio and driven pulley axial thrust, and the CVT is controlled in such a manner that the driven pulley axial thrust is kept at the peak of the thrust ratio.
Specifically, the reference is based on a finding that the thrust ratio peaks slightly before the occurrence of a large belt slippage, so that the CVT should preferably controlled to keep the driven pulley thrust at its peak.
Apart from the above, another U.S. Pat. No. 5,964,818 (corresponding to a Japanese Laid-Open Patent application No. Hei10(1998)-89429) proposes a technique to calculate friction coefficients between the pulley and belt by presuming the friction coefficients of the drive and driven pulleys as appropriate values and by measuring rotational speed and input torque of the drive pulley, an axial thrust of the driven pulley, and an inter-shaft force between the input and output shafts, etc., to substituting them into relational expressions for equilibrium of force each describing parts of the CVT.